The present invention relates to the field of joining technology. It relates to a method for welding together two parts which are exposed to different temperatures in accordance with the preamble of claim 1.
The efficiency of a thermal machine, for example in a steam power plant, can be improved considerably by increasing the working temperature, for example to over 650xc2x0 C. The rotors of the high-temperature steam turbines used in a power plant of this type, for working temperatures of over 650xc2x0 C., should be produced from creep-resistant nickel-base alloys. However, nickel-base alloys (superalloys) of this type, such as for example Inconel, are considerably more expensive (for example ten times more expensive) than standard steels.
On the other hand, in thermal machines there are sections which are exposed to very high temperatures and sections which are exposed to less high temperatures. To minimize the costs of a turbine rotor or similar machine parts, it would therefore be possible and desirable to produce the high-temperature part from a nickel-base alloy and the remainder (e.g. the end pieces of the rotor) from steel. Producing a rotor of this type from individual rotor discs, as is described, for example, in U.S. Pat. No. 4,086,690 then requires the parts made from steel and the parts made from nickel-base alloy to be joined. A join of this type can be produced either by screw connections or by welding. Since in large machines screw connections are exposed to extremely high loads and there is scarcely any experience of such connections in machines of this type, in actual fact the only suitable joining process is welding.
Suitable nickel-base alloys for applications of this type may be alloyed with Nb in order to acquire the required resistance to creep. An Nb-containing nickel-base alloy of this type is known, for example, under the name Inconel 625 or IN625. Parts which consist of alloys of this type can easily be joined together by welding.
On the other hand, welded joints between parts which consist of steel and parts which consist of a nickel-base alloy which contains, for example, Nb or other additional elements cause problems. The local melting of the nickel-base alloy during welding in this case leads to an increase in the levels of Nb or the other additional element in the weld seam, leading to the formation of cracks in the welded joint. Attempts to reduce the extent to which the nickel-base alloy is melted, by reducing the introduction of heat by using relatively thin welding electrodes or the like, have hitherto enjoyed only limited success.
It is known from U.S. Pat. No. 4,962,586 to form a turbine rotor by welding together two rotor parts which are exposed to different temperatures and consist of two different low-alloy steels. To improve the welded joint, first of all surfacing welding is used to apply a layer of filler material to the joining surface of the rotor part which is intended for high temperatures, and this layer is then machined. The filler material leads to better matching between the different steel grades in the welded joint. Then, the two parts are welded together. This known process differs in that it relates to the operation of joining two parts made from steel. The cladding (FIG. 3, 4) is carried out so that the clad part (clad layer 51) can be more highly annealed than is tolerable for the other part. The procedure is thermally managed and comprises the steps of cladding, high annealing, welding and low annealing.
It is an object of the invention to provide a method for welding together parts made from steel and from a nickel-base alloy which contains additional elements, such as for example Nb, which, while being simple to employ and involving little outlay, leads to a stable welded joint, and to provide a turbomachine which is produced using this method.
The object is achieved by the combination of features given in claims 1 and 9. The essence of the invention consists in, first of all, applying an intermediate layer, in which the amount of the additional elements, such as for example Nb, is progressively reduced from the inside outward, to the joining surface, which is provided for welding, of the part which consists of the superalloy which contains, for example, Nb, and then welding the part which has been provided with the intermediate layer to the part which consists of steel. The intermediate layer reduces the harmful increase in the levels of Nb or other elements which are unfavorable to welding in the weld seam to such an extent that a stable, crack-free welded joint is formed.
According to a first preferred configuration of the method according to the invention, the intermediate layer is composed of a nickel-base alloy which is free of the additional elements. This results in a high-quality welded joint.
The production of the intermediate layer is made particularly simple if the intermediate layer, according to another configuration of the invention, is composed of a plurality of single layers which are applied in succession and lie on top of one another. In this case, it has been found that, for the intermediate layer, five single layers of IN617 on an IN625 part which is to be welded are sufficient to reliably prevent cracking of the welded joint if further layers from steel electrodes are used for the welding.
The number of the single layers in the intermediate layer can be reduced by carrying out the welding using the MAG (metal active gas) process or the TIG (tungsten inert gas) process. Application by means of strip electrodes, wire electrodes, using the electroslag strip process or by casting on is also conceivable.
It is preferable for the second part to consist of Inconel 625 (IN625) and for the intermediate layer or the single layers to be formed from Inconel 617 (IN617).
A preferred configuration of the turbomachine according to the invention is characterized in that the end pieces and the adjacent rotor discs or drums are joined to one another with a material-to-material bond by a weld seam, and in that an intermediate layer of a nickel-base alloy, preferably Inconel 617 (IN617), in which the amount of the additional elements is progressively reduced toward the weld seam (19), is arranged between the weld seam and the respective rotor disc or drum.
Further embodiments will emerge from the dependent claims.